In a first kind of prior art reflow oven heat is applied to one side of the board.
In a second kind of prior art reflow oven heat is supplied to both sides of the board. In this type of oven the temperature applied to each side of the board is substantially equal.
When reflow-soldering printed circuit boards containing components like connectors or sockets, which have pins, the pins extend through holes in the circuit board. Solder paste applied to one side of the board prior to the insertion of such components is usually dragged along with the pins during the insertion process and ends up on both sides of the board.
During the reflow process, the solder paste is melted, and the molten solder flows over the pins and circuit board. When the board is cooled and the solder solidifies the connection is made. However, during the reflow process gases are liberated from the melting reflow solder. Bubbles of such gases may lead to voids, especially within the holes of the circuit board and adjacent to the pins of through-hole components. This deteriorates the soldering quality.